This invention relates to a rotation detector for use with an electric motor and in particular, to a rotation detector for a miniature permanent magnet direct current (PMDC) motor.
Many applications of PMDC motors require the speed of the motor to be monitored or the number of revolutions made by the motor shaft to be counted. This is often achieved using a Hall sensor and a magnet fitted to the shaft or by using an encoder fitted to the shaft in co-operation with an optical sensor. Both of these methods involve relatively expensive parts, are time consuming to fit and require the motor to be designed to accommodate these parts as they occupy a significant amount of space either inside the motor, increasing the size of the motor or outside the motor where they are vulnerable to damage.
Recently, rotation detectors which use an induction coil to monitor changes in the magnetic flux of the motor as the rotor rotates have been developed, see for example, EP-A-0626748 in which an inductor/capacitor circuit fitted into a separate case which is fitted to the non-drive end of the motor by being press fitted to the bearing boss. In this arrangement, the coil is detecting leakage flux and for efficient motors, it is hoped that flux leakage is minimised to increase efficiency, reducing the reliability/sensitivity of this type of detector. See also EP-A-0529131 which uses a coil mounted within the motor.